The volumetric adsorption method of measuring the pore volume and surface area of porous samples has gained wide acceptance in industry. Instruments for carrying out such analyses are described, for example, in U.S. Pat. No. 3,850,040; and in U.S. Pat. No. 4,566,326. In advanced instruments, each sample is dosed with amounts of an adsorbate gas, such as nitrogen, in order to determine how much of the gas is adsorbed on the surface of the pores of the sample at selected relative pressures P/P.sub.o, that is, at certain values of pressure P within the sample chamber relative to the saturation pressure P.sub.o of the gas at the temperature of the sample. The sample is usually maintained at the temperature of liquid nitrogen. Computer controllers have been developed in order to control the dosing of the sample from a manifold to bring the sample pressure up to each target pressure without overshooting it. Control algorithms of this type are described in both of the patents referenced above.
In order to know when target pressures have been reached, the system must be provided with a value for the saturation pressure of the adsorbate gas. Several techniques for determining the saturation pressure P.sub.o have been utilized. In the simplest, P.sub.o is assumed to be equal to atmospheric pressure plus 5-15 Torr. This may be sufficiently accurate for single point BET methods in which the saturation pressure value does not strongly influence the result. However, other measurements, such as obtaining a desorption isotherm for accurate determination of pore volume, require an accurate value for P.sub.o, which is very dependent upon temperature.
P.sub.o has also been measured in a different liquid nitrogen (LN.sub.2) bath than that in which the sample chamber is immersed. This technique has been utilized in a commercial instrument of the type described in U.S. Pat. No. 4,566,326, by devoting one of the sample stations to an empty cell immersed in its own LN.sub.2 bath. The empty cell is filled with nitrogen gas which is liquified, and the vapor pressure is monitored and used as P.sub.o. However, the use of a different LN.sub.2 bath from that in which the sample is immersed leads to significant errors in the P.sub.o value because of variations in the temperature of the baths.
Another technique is described in U.S. Pat. No. 3,850,040. A separate P.sub.o tube, containing a quantity of suitable material having a large surface area and an amount of adsorbate gas sufficient to condense the gas on the exterior of the material, is immersed with the sample tube into the same LN.sub.2 bath. This technique avoids the problem arising from the use of different baths that may vary in temperature, but does not take into account the effect of the sample itself on P.sub.o, and therefore is not as accurate as the technique of the present invention, described below.
Finally, P.sub.o has been measured in a different manner a flowing gas adsorption system as described in U.S. Pat. No. 3,555,912. The pressure gauge that is used to indicate the pressure over the sample due to down stream impedance is also used to measure the equilibration vapor pressure P.sub.o of the adsorbate. The entire system is purged with adsorbate, the sample cell is immersed in LN.sub.2, and pressure is allowed to build up in the system in order to liquify the adsorbate in the sample cell. The sample cell is isolated, and valves opened to cause the liquid adsorbate to boil, whereby the vapor will flow through the gauge to purge it. Then the purging valve associated with the gauge is closed and the gauge reading taken as P.sub.o. By its nature, this technique lacks control over the temperature at which the P.sub.o value is acquired. The flowing adsorbate gas entering the measurement area brings in heat and therefore requires an elevated pressure to liquify, while the vigorous boiling which causes the flow of gas to the gauge can lead to temperature variations caused by uncontrolled cooling. There is no control of the amount of liquid that forms, and the amount may be such that its temperature is different from the temperature of LN.sub.2 in the bath. When the flow of gas is reestablished for analysis of the sample, the temperature will vary from that at the time P.sub.o is measured. As a result of these variable factors, this system provides an unreliable measurement of P.sub.o.
Thus, it will be seen that there has been a need for a technique and apparatus for obtaining accurate P.sub.o values representing the saturation pressure under conditions essentially identical to those present as the sample is receiving doses of the adsorbate gas during analysis.
Another problem encountered during volumetric sorption analysis is specific to porous materials which adsorb large volumes of adsorbate gas at low relative pressures. This characteristic is typical of materials such as Zeolites and other Type 1 materials, although some Type 2 and Type 4 materials also exhibit large uptake of gas upon initial exposure to the gas. Operators of manual sorption analyzers had the opportunity to guess based upon experience that a particular material might be able to adsorb a large dose of gas without exceeding the first target relative pressure. However, automatic instruments prior to this invention have been incapable of differentiating between relative pressures at which adsorption is large, versus those at which little adsorption occurs. Typically, such instruments have calculated the size of all doses to bring the sample pressure up to the target relative pressure by calculating the gas needed to fill the free space around the sample at the target relative pressure, plus an allowance for adsorption by the sample. This allowance for adsorption must be limited, however, so as not to overshoot the target relative pressures in regions where little additional adsorption occurs. The system described in U.S. Pat. No. 3,850,040 allows the user to elect to increase the size of the dosing manifold by a fixed additional volume, but this would then apply to all doses, not just those at relative pressures at which large adsorption by the sample occurs.
As a result, given a sample that adsorbs large quantities of gas at low relative pressures, over one hundred small doses must sometimes be made by the automatic instrument in order to reach the initial target relative pressures. This repetitive and unproductive procedure wastes time and causes great wear on valves, seals and other components of the gas containing hardware.
Thus, there has been a need for an apparatus and method for controlling dosing in volumetric sorption analyzers capable of avoiding the need for an excessive number of doses in the analysis of Type 1 and similar materials.